1. Field of the Invention
The present invention relates to a power transmission shaft coupled to a constant velocity joint for torque transmission.
2. Description of the Related Art
A power transmission shaft combined with a constant velocity joint is employed in various mechanical parts of an automobile, an industrial machine and so on. Such kind of power transmission shaft is usually coupled with a mating member (boss) fitted around an outer circumference thereof via a serration or a spline, for transmitting a torque to the mating member. The serrated shaft or the spline shaft is made of a medium carbon steel or a low-alloy steel, subjected to a heat treatment such as a surface hardening treatment or a thermal refining, more specifically a induction hardening, carburized quenching, nitriding, etc. for increasing a shaft strength, taking into consideration the plastic workability, mechanical workability and cost. The Japanese patent publication No. 3194093 discloses a technique for improving a shaft strength by a heat treatment, wherein a induction hardening and a carburized quenching is applied to a shaft-shaped mechanical part, so that a ratio of an effective depth of hardening against the part diameter is set in a range of 0.4 to 0.8.
According to the cited publication, the heat treatment is applied to the part without particular distinction of a serrated portion for torque transmission such as a spline and a smooth portion on a cylindrical surface where such serration is not formed. This can be construed as a result of performing the heat treatment simultaneously and under a same condition, on both of the serrated portion for torque transmission and the smooth portion, in which case a depth of hardening becomes substantially the same in the serrated portion for torque transmission and the smooth portion.
In recent years, along with the increasing social concern for the global environment, in the field of the automobile industry for example, more stringent emission control and improvement of fuel efficiency have come to be more severely demanded. In order to cope with such trend, there has been arising a strong demand for a lighter in weight yet stronger power transmission shaft, such as a propeller shaft or a driveshaft. Under such severe industrial circumstances, the invention according to the above-cited publication may not be able to fully meet the requirements.
Accordingly, it is an object of the present invention to provide a technique for improving a strength, especially a fatigue strength of a power transmission shaft provided with a serrated portion for torque transmission such as a spline shaft or a serrated shaft.